Liquid crystal display apparatus

ABSTRACT

This invention is directed to a liquid crystal display apparatus which comprises a liquid crystal display element, and a driving device for driving the display element by simple matrix driving. The display element includes a liquid crystal layer exhibiting a cholesteric phase, and has scanning electrodes and signal electrodes. The driving device is configured such that a driving voltage of single polarity including a selection signal voltage is applied to the scanning electrodes in each frame, and the polarity of the driving voltage is reversed in every frame. The scanning electrodes are successively brought to a selected state by applying the selection signal voltage to each scanning electrode in a scanning period set for the scanning electrode, while a rewriting signal voltage corresponding to each scanning electrode in the selected state is applied to each signal electrode. An application period of the selection signal voltage to the scanning electrode is ½ the scanning period.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based on Japanese patent applicationNo.2002-87192 filed in Japan on Feb. 18, 2002, the entire content ofwhich is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method of driving a liquidcrystal display element, a device for driving a liquid crystal displayelement and a liquid crystal display apparatus.

[0004] 2. Description of Related Art

[0005] Usually the liquid crystal display apparatus includes a liquidcrystal display element and a device for driving the liquid crystaldisplay element.

[0006] The liquid crystal display element basically comprises a pair ofsubstrates and a liquid crystal layer disposed between the substrates.By applying a predetermined driving voltage to the liquid crystal layer,the arrangement of liquid crystal molecules is controlled so that lightincident on the liquid crystal element is modulated to perform a desiredimage display.

[0007] A wide variety of liquid crystal display elements have beenproposed. In recent years, research has been conducted on a liquidcrystal display element including a chiral nematic liquid crystalcomposition prepared by adding a chiral material to a nematic liquidcrystal, the composition being caused to exhibit a cholesteric phase atroom temperature due to the chiral material.

[0008] This type of liquid crystal display element is usable, forexample, as a liquid crystal display element of reflection typeutilizing a selective reflection capability of the chiral nematic liquidcrystal composition.

[0009] In the reflection type liquid crystal display element, an imagedisplay can be performed by applying a high or low pulse voltage toswitch the liquid crystal composition to a planar state (colored state)or to a focal conic state (transparent state).

[0010] Even after stopping the application of such pulse voltage, theliquid crystal composition can be kept in the planar state or the focalconic state, in other words, the liquid crystal composition can exhibitthe so-called bi-stable property or can achieve a memory effect, wherebythe image display can be kept after stopping the application of voltage.

[0011] The reflection type liquid crystal display element can perform amonochromatic (mono-color) image display utilizing a black or similarcolor background, a 2-color image display or a full color image display.

[0012] To realize, for example, an image display in full color, it ispossible to use a laminate type liquid crystal display element includingat least three liquid crystal layers, i.e. a red liquid crystal layerwhich can perform a red display, a green liquid crystal layer which canperform a green display, and a blue liquid crystal layer which canperform a blue display.

[0013] When at least one liquid crystal layer of this laminate typeliquid crystal display element is maintained in a planar state (coloredstate), red, green, blue or other color can be displayed. When thelaminate type liquid crystal display element is maintained in a focalconic state (transparent state), black color or like background colorcan be displayed.

[0014] In the liquid crystal display elements, usually electrodes areformed on the pair of substrates between which the liquid crystal layeris held, and are disposed so that the electrode-forming surfaces of thesubstrates are opposed to each other.

[0015] For example, a liquid crystal display element has an imagedisplay region composed of a plurality of pixels which are driven by amatrix driving system using a plurality of scanning electrodes and aplurality of signal electrodes which are opposed to each other.

[0016] In this liquid crystal display element, for example, a pluralityof strips of scanning electrodes (or signal electrodes) extend in apredetermined direction with a specified gap in parallel with each otheron one of the paired substrates, while a plurality of strips of signalelectrodes (or scanning electrodes) extend in a predetermined directionwith a specified gap in parallel with each other on the other substrate.The two groups of electrodes extend across each other when viewed from aplane. Each of the pixels corresponds to a portion of electrodes whichintersect each other on the paired substrates.

[0017] Each electrode formed on the paired substrates is connected to adevice for driving the liquid crystal display element. When apredetermined driving voltage is applied to the electrodes on thesubstrates from the driving device connected to the electrodes, theliquid crystal is driven to display a desired image.

[0018] The liquid crystal display element can be driven, for example, bya simple matrix driving method.

[0019] In the simple matrix driving method, the device for driving theliquid crystal display element includes, for example, a scanning drivingIC connected to the plurality of scanning electrodes and capable ofsupplying a predetermined selection signal voltage to the scanningelectrodes and a signal driving IC connected to the plurality of signalelectrodes and capable of supplying a predetermined rewriting signalvoltage to the signal electrodes.

[0020] The scanning electrodes are successively brought to a selectedstate by successively applying the predetermined selection signalvoltage to each scanning electrode from the scanning driving ICconnected to the plural scanning electrodes, while applying thepredetermined rewriting signal voltage to each signal electrode insynchronization with application of the selection signal voltage to eachscanning electrode from the signal driving IC connected to the pluralsignal electrodes to apply to the liquid crystal a voltage correspondingto a potential difference between the selection signal voltage and therewriting signal voltage, whereby the liquid crystal is driven.

[0021] When the liquid crystal is driven by such simple matrix drivingmethod, a voltage to be applied to the liquid crystal may be, forexample, an alternating voltage which undergoes a periodical change inpolarity of voltage waveform in each frame (for example, rectangularpulse voltage involving a periodical change in polarity of voltagewaveform) from the viewpoint of increase in lifetime of the liquidcrystal and others.

[0022] However, when an alternating voltage involving a periodicalchange in polarity of voltage waveform is applied to the liquid crystalwhich serves also as a condenser, a current more easily flows betweenthe electrodes having the liquid crystal therebetween with an increasein the waveform repeating frequency of such alternating voltage, inwhich case the consumed power is increased for driving the liquidcrystal display element.

[0023] Further, since a voltage corresponding to the alternating voltageis supplied to the scanning driving IC in one frame, the scanningdriving IC is required to have a capability of withstanding a voltagecorresponding to a difference between maximum and minimum voltages inthe alternating electric field.

[0024] In the simple matrix driving method, a rewriting signal voltageis applied to the signal electrode corresponding to a pixel to bedisplayed in synchronization with the selection signal voltage in eachscanning time in which the selection signal voltage is applied to thescanning electrode, and a voltage is applied based on the selectionsignal voltage and the rewriting signal voltage to the liquid crystalcorresponding to the pixel to be displayed. In this operation, a voltageis applied to the liquid crystals corresponding to pixels not to bedisplayed by the rewriting signal voltage. Namely the so-called“cross-talk” occurs.

[0025] Due to the cross-talk in the the liquid crystal corresponding tothose pixels, for example, one or more of the pixels to be displayed inhigh density are displayed in slightly low density, or one or more ofthe pixels to be displayed in low density are displayed in slightly highdensity. Namely an image like a shadow appears in the foregoing pixelportions. In other words, the phenomenon of shadowing occurs.

SUMMARY OF THE INVENTION

[0026] An object of the present invention is to provide a liquid crystaldisplay apparatus which comprises a liquid crystal display elementhaving a liquid crystal, and a driving device for driving the liquidcrystal display element by matrix driving, the liquid crystal displayapparatus being capable of reducing the consumption of power for drivingthe liquid crystal display element.

[0027] Another object of the present invention is to provide a liquidcrystal display apparatus which comprises a liquid crystal displayelement having a liquid crystal, and a driving device for driving theliquid crystal display element by matrix driving, the liquid crystaldisplay apparatus being capable of suppressing shadowing from occurringin image display when the liquid crystal suffers cross-talk due to arewriting signal voltage applied to a signal electrode to display abetter image.

[0028] A further object of the present invention is to provide a liquidcrystal display apparatus which comprises a liquid crystal displayelement having a liquid crystal, and a driving device for driving theliquid crystal display element by matrix driving, the liquid crystaldisplay apparatus being capable of employing a driving IC, which is lowin voltage resistance, for driving the liquid crystal display element.

[0029] The present invention provides the following liquid crystaldisplay apparatuses.

[0030] (1) First Liquid Crystal Display Apparatus

[0031] A liquid crystal display apparatus comprising:

[0032] a liquid crystal display element that includes a layer of aliquid crystal exhibiting a cholesteric phase, and a plurality ofscanning electrodes and a plurality of signal electrodes extendingacross each other with the liquid crystal layer therebetween forperforming display utilizing a selective reflection capability of theliquid crystal; and

[0033] a driving device for driving the liquid crystal display elementby simple matrix driving,

[0034] wherein the driving device is configured such that (1) a drivingvoltage of single polarity including a selection signal voltage isapplied to the scanning electrodes in each frame, and the polarity ofthe driving voltage is reversed in every frame; (2) the scanningelectrodes are successively brought to a selected state by applying theselection signal voltage to each scanning electrode in a scanning periodset for the scanning electrode, while a rewriting signal voltagecorresponding to each scanning electrode in the selected state isapplied to each signal electrode in synchronization with application ofthe selection signal voltage to the scanning electrode; and (3) anapplication period of the selection signal voltage to the scanningelectrode is ½ the scanning period (half of the scanning period).

[0035] (2) Second Liquid Crystal Display Apparatus

[0036] A liquid crystal display apparatus comprising:

[0037] a liquid crystal display element that includes a layer of aliquid crystal exhibiting a cholesteric phase, and a plurality ofscanning electrodes and a plurality of signal electrodes extendingacross each other with the liquid crystal layer therebetween; and

[0038] a driving device for driving the liquid crystal display elementby simple matrix driving,

[0039] wherein the driving device is configured such that (1) a drivingvoltage of single polarity including a selection signal voltage, a resetvoltage and a maintaining voltage is applied to the scanning electrodesin each frame, and the polarity of the driving voltage is reversed inevery frame; (2) the scanning electrodes are successively brought to aselected state by applying the selection signal voltage to each scanningelectrode in a scanning period set for the scanning electrode, while arewriting signal voltage corresponding to each scanning electrode in theselected state is applied to each signal electrode in synchronizationwith application of the selection signal voltage to the scanningelectrode; (3) reset voltage is applied to the scanning electrode tobring the liquid crystal to a homeotropic state before applying theselection signal voltage, and the maintaining voltage is applied to thescanning electrode to establish a state of the liquid crystal to beselected by the selection signal voltage after applying the selectionsignal voltage; and (4) the rewriting signal voltage to be applied tothe signal electrode is changed in polarity within the scanning period,and effective values of positive voltage and negative voltage of therewriting signal voltage are substantially equal to each other withinthe scanning period.

[0040] (3) Third Liquid Crystal Display Apparatus

[0041] A liquid crystal display apparatus comprises a liquid crystaldisplay element, and a driving device for driving the liquid crystaldisplay element by simple matrix driving, wherein the liquid crystaldisplay element includes a liquid crystal layer, and has a plurality ofscanning electrodes and a plurality of signal electrodes extendingacross each other with the liquid crystal layer therebetween, whereinthe driving device is configured such that a driving voltage of singlepolarity including a selection signal voltage is applied to the scanningelectrodes in each frame, and the polarity of the driving voltage isreversed in every frame; the scanning electrodes are successivelybrought to a selected state by applying the selection signal voltage toeach scanning electrode in a scanning period set for the scanningelectrode, while a rewriting signal voltage corresponding to eachscanning electrode in the selected state is applied to each signalelectrode in synchronization with application of the selection signalvoltage to the scanning electrode; and the rewriting signal voltage tobe applied to the signal electrode is changed in polarity within thescanning period, and effective values of positive voltage and negativevoltage of the rewriting signal voltage are substantially equal to eachother within the scanning period.

[0042] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043]FIG. 1 is a sectional view schematically showing a structure of areflective/laminate type full-color liquid crystal display element whichcan be driven by simple matrix driving method.

[0044]FIG. 2 is a block diagram showing an example of a driving circuitwhich is a main part of a driving device which applies driving voltagesto the liquid crystal display layer.

[0045]FIG. 3 shows an example of a detailed structure of the drivingcircuit shown in FIG. 2.

[0046]FIG. 4 shows another example of a detailed structure of thedriving circuit shown in FIG. 2, and shows a state of odd-numbered frame(plus frame) in which switching elements are changed over to a side 1.

[0047]FIG. 5 shows a state of even-numbered frame (minus frame) in thecircuit shown in FIG. 4, in which the switching elements are changedover to a side 2.

[0048]FIG. 6(A) shows a basic driving waveform which is output from ascanning driving IC to each scanning electrode in the odd-numberedframes, and FIG. 6(B) shows a basic driving waveform which is outputfrom the scanning driving IC to each scanning electrode in theeven-numbered frames.

[0049]FIG. 7 shows waveforms of voltages which are output from thescanning driving IC to the scanning electrodes, a waveform of voltagewhich is output from a signal driving IC to one of signal electrodes,and waveforms of voltages which are applied to liquid crystalscorresponding to pixels, in one of the odd-numbered frames.

[0050]FIG. 8 shows waveforms of voltages which are output from thescanning driving IC to the scanning electrodes, a waveform of voltagewhich is output from the signal driving IC to one of the signalelectrode, and waveforms of voltages which are applied to the liquidcrystals corresponding to pixels, in one of the even-numbered frames.

[0051]FIG. 9 shows a waveform of selection pulse which is output to oneof row electrodes (scanning electrodes), a waveform of signal pulsewhich is output to one of column electrodes (signal electrodes) and awaveform applied to the liquid crystal by these voltages for finallyselecting the liquid crystal in a maximum selective reflection state, inone of the odd-numbered frames.

[0052]FIG. 10 shows a waveform of selection pulse which is output to oneof the row electrodes, a waveform of signal pulse which is output to oneof the column electrodes and a waveform which is applied to the liquidcrystal by these voltages for finally selecting the liquid crystal in anintermediate tone display state, in one of the odd-numbered frames.

[0053]FIG. 11 shows a waveform of selection pulse which is output to oneof the row electrodes, a waveform of signal pulse which is output to oneof the column electrodes and a waveform which is applied to the liquidcrystal by these voltages for finally selecting the liquid crystal in atransparent state, in one of the odd-numbered frames.

[0054]FIG. 12(A), FIG. 12(B), and FIG. 12(C) show enlarged portions,chiefly in selection periods, shown in FIG. 10, FIG. 9 and FIG. 11, of awaveform of selection pulse which is output to the row electrode, awaveform of signal pulse which is output to the column electrode, and awaveform applied to the liquid crystal by these voltages.

[0055]FIG. 13(A) to FIG. 13(C) show an example of signal pulse whereincontinuous time T1 or T2 of positive or negative voltage is not the sameas the selection signal application period Tsp.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] The liquid crystal display apparatus according to a preferredembodiment of the present invention basically comprises a liquid crystaldisplay element, and a driving device for driving the liquid crystaldisplay element. The liquid crystal display element may include a layerof a liquid crystal exhibiting a cholesteric phase (cholestericcharacteristics), and may perform display utilizing a selectivereflection of the liquid crystal. The element has a plurality ofscanning electrodes and a plurality of signal electrodes, theseelectrodes extending across each other with the liquid crystal layertherebetween. The driving device can drive the liquid crystal element bysimple matrix driving.

[0057] The driving device is configured such that a driving voltage ofsingle polarity including a selection signal voltage is applied to thescanning electrodes in each frame, and the polarity of the drivingvoltage is reversed in every frame.

[0058] The driving device is also configured such that the scanningelectrodes are successively brought to a selected state by applying theselection signal voltage to each scanning electrode in a scanning periodset for the scanning electrode, while a rewriting signal voltagecorresponding to each scanning electrode in the selected state isapplied to each signal electrode in synchronization with application ofthe selection signal voltage to the scanning electrode. An applicationperiod of the selection signal voltage to the scanning electrode may be½ the scanning period (half of the scanning period). The scanning periodis a period in which the selection signal voltage is applied to thescanning electrode.

[0059] In the liquid crystal display apparatus, the driving voltage tobe applied to the scanning electrode for scanning in each frame formatrix driving of the liquid crystal display element is given a singlepolarity and the polarity is reversed in every frame so that the stateof single polarity of voltage applied to the liquid crystal in eachframe can continuously last for a prolonged period of time. Consequentlyit is possible to reduce a practical waveform repeating frequency ofvoltage to be applied to the liquid crystal as compared with use ofalternating voltage which periodically changes in polarity of waveformof voltage in each frame.

[0060] Further it is possible to decrease a value of the driving voltageto be applied to the scanning electrode to ½, and an amount of powerconsumed for driving the liquid crystal display element can becorrespondingly reduced. Namely the power consumption for driving theliquid crystal display element can be decreased. Moreover, a scanningdriving IC which is inexpensive and is relatively low in voltageresistance can be used in view of reduction of the voltage to besupplied to the scanning driving IC to ½ compared with the use ofalternating voltage.

[0061] Examples of the liquid crystal to be used for the liquid crystaldisplay element which exhibit a cholesteric phase include those whichexhibit a cholesteric phase (cholesteric characteristics) at roomtemperature (e.g. about 25° C.). The liquid crystals exhibiting acholesteric phase include, for example, a cholesteric liquid crystalcapable of showing a cholesteric phase by itself and a chiral nematicliquid crystal composition prepared by adding a chiral material to anematic liquid crystal. The chiral nematic liquid crystal compositioncan selectively reflect light in a predetermined wavelength range andcan achieve a memory effect. The selective reflection wavelength can beadvantageously adjusted by changing the amount of chiral material to beadded.

[0062] The driving device for driving the liquid crystal display elementmay comprise a scanning driving IC to be connected to the plurality ofscanning electrodes, a signal driving IC to be connected to theplurality of signal electrodes, and a controller for controlling thesedriving ICs. The controller may be adapted to control the scanningdriving IC such that a selection signal voltage is successively appliedto the scanning electrodes to bring the electrodes to a selective state,while it may be adapted to control the signal driving IC such that arewriting signal voltage is applied to each signal electrode, morespecifically a rewriting signal voltage corresponding to the scanningelectrode in the selective state is applied to the signal electrode insynchronization with application of the selection signal voltage to eachscanning electrode.

[0063] The controller may control the scanning driving IC in a mannersuch that the driving voltage to be applied to the scanning electrode inscanning in each frame for matrix driving of the liquid crystal displayelement is given a single polarity in each frame and inversion of thepolarity is performed in every frame.

[0064] In order to properly drive the liquid crystal exhibiting acholesteric phase in each pixel, a predetermined reset voltage may beapplied to the scanning electrode for a specified time period (resetperiod) to bring the liquid crystal to a homeotropic state before apredetermined period (selection period) of applying the selection signalvoltage. In this case, the selection signal voltage is a sufficientvoltage to change the homeotropic liquid crystal to a desired state.

[0065] A predetermined maintaining voltage for establishing a state ofthe liquid crystal to be selected by the selection signal voltage may beapplied to each scanning electrode for a specified period (maintainingperiod) after applying the selection signal voltage.

[0066] In the selection period for applying the selection signalvoltage, the driving voltage for the scanning electrode may be 0V for aspecified period (pre-selection period) after the reset period forapplying the reset voltage and before applying the selection signalvoltage, and the driving voltage for the scanning electrode may be 0Vfor a specified period (post-selection period) before the maintainingperiod for applying the maintaining voltage and after applying theselection signal voltage.

[0067] When each scanning electrode is brought to a selective state bysuccessively applying the selection signal voltage to the plurality ofscanning electrodes while applying the rewriting signal voltagecorresponding to the scanning electrode in the selective state insynchronization with application of the selection signal voltage to thescanning electrode, the rewriting signal voltage may be such that therewriting signal voltage is changed in polarity within the scanningperiod and effective values of positive and negative voltages of therewriting signal voltage is substantially equal to each other within thescanning period.

[0068] With these features, when the liquid crystals corresponding tothe pixels are subjected to a cross-talk due to the rewriting signalvoltage applied to the signal electrode, voltages applied to the liquidcrystals due to the cross-talk can be rendered substantially uniform.

[0069] However, in this case, if the application period of the selectionsignal voltage is as long as the scanning period, the pixel to bedisplayed is not be properly displayed by the rewriting signal voltagewhich is changed in its polarity in the scanning period.

[0070] In view of the above, it is recommendable that the applicationperiod of the selection signal voltage to be applied to the scanningelectrodes is ½ the scanning period when applying the driving voltagesto the plurality of scanning electrodes and the plurality of signalelectrodes for matrix driving.

[0071] The pixels to be displayed can be properly displayed, if thefollowing features are realized: an application period of the selectionsignal voltage is ½ the scanning period; the rewriting signal voltage ischanged in its polarity within the scanning period; the effective valuesof positive and negative voltages of the rewriting signal voltage aresubstantially equal to each other within the scanning period; each oftotal of period(s) of the positive voltage and total of period(s) of thenegative voltage is as long as the application period of the selectionsignal voltage.

[0072] The following also results from such features. When liquidcrystals corresponding to pixels are subjected to the cross-talk due tothe rewriting signal voltage applied to the signal electrode, thevoltages applied to the liquid crystals corresponding to pixels due tothe cross-talk can be rendered substantially uniform. Thereby it ispossible to suppress a shadowing in image display from occurring in theliquid crystals subjected to the cross-talk, resulting in better imagedisplay. Furthermore, each pixel can be displayed in a planar state(selective reflective state), in a focal conic state (transparent state)or in an intermediate tone state (mixed states), for example, byshifting the phase of the rewriting signal voltage.

[0073] In the liquid crystal display apparatus and the device fordriving the liquid crystal display element which have the scanningdriving IC, the signal driving IC and the controller, the controller maybe adapted to control the signal driving IC so as to adjust theapplication period of the selection signal voltage to ½ the scanningperiod, and may be adapted to control the signal driving IC so as tochange the polarity of the rewriting signal voltage within the scanningperiod, so as to substantially equalize the effective values of positiveand negative voltages with each other in the scanning period, and so asto allow the rewriting signal voltage to equalize each of total ofperiod(s) of the positive voltage and total of period(s) of the negativevoltage with the application period of the selection signal voltagewithin the scanning period.

[0074] At any rate, it is desired to equalize an application period ofthe rewriting signal voltage with the scanning period as far as therewriting signal voltage is concerned in which the polarity thereof ischanged within the scanning period, the effective values of positive andnegative voltages are substantially equal to each other in the scanningperiod, and each of total of period(s) of the positive voltage and totalof period(s) of the negative voltage is made equal to the applicationperiod of the selection signal voltage within the scanning period.

[0075] A rectangular pulse voltage which has a duty ratio of 50% in thescanning period and in which the absolute values of positive andnegative voltages are identical with each other can be mentioned as atypical example of the rewriting signal voltage in which the polarity ischanged in the scanning period, the effective values of positive andnegative voltages are substantially equal to each other in the scanningperiod, each of the total of period(s) of the positive voltage and thetotal of period(s) of the negative voltage is equal to the applicationperiod of the selection signal voltage and the application period of therewriting signal voltage is as long as the scanning period.

[0076] When the selection signal voltage is such that its applicationperiod is ½ the scanning period, and the rewriting signal voltage is,for example, a rectangular pulse voltage which has a duty ratio of 50%within the scanning period and in which the absolute values of positiveand negative voltages are identical with each other, voltages to beapplied to the liquid crystals corresponding to pixels due to thecross-talk can be made substantially constant, whereby the shadowingoccurring due to the cross-talk in image display can be furthersuppressed. This matter will be described in greater detail later.

[0077] At any rate, the phase of the signal voltage may be adjusted tobring about the following: the application period of the selectionsignal voltage is ½ the scanning period; the rewriting signal voltagechanges its polarity within the scanning period; the effective values ofpositive and negative voltages of the rewriting signal voltage aresubstantially equal to each other within the scanning period; and therewriting signal voltage is such that each of the total of period(s) ofthe positive voltage and the total of period(s) of the negative voltageis as long as the application period of the selection signal voltagewithin the scanning period.

[0078] The voltage to be applied to the liquid crystal exhibiting acholesteric phase can be changed by adjusting the phase of the signalvoltage so that the liquid crystal can be brought to a planar state(selective reflective state), to a focal conic state (transparent state)or to a mixed state (a planar state and a focal conic state are mixed).Thereby the liquid crystal element is allowed to perform a selectivereflective (colored) display, a transparent display or an intermediatetone display.

[0079] At any rate, the driving voltage may be applied to the scanningelectrode by the scanning driving IC connected to a power source whichcan switch positive and negative of output voltage, and can switch thepositive and negative of power source output voltage in every frame.Thereby, the driving voltage to be applied to the scanning electrode canbe given a single polarity in each frame and the polarity can bereversed in every frame. In this way, a driving of the liquid crystalelement can be realized by simple circuit structure.

[0080] The liquid crystal apparatus and the device for driving theliquid crystal display element, which have the scanning driving IC, thesignal driving IC and the controller, may be provided with a powersource which is connected to the scanning driving IC and which canswitch positive and negative of output voltage, and the driving voltagemay be applied to the scanning electrodes by the scanning driving ICconnected to the power source.

[0081] In this case, the controller may control the power source and thescanning driving IC such that the power output voltage is switched frompositive to negative or negative to positive in every frame, whereby thedriving voltage to be applied to the scanning electrode is given asingle polarity in each frame and the polarity is reversed in everyframe.

[0082] The liquid crystal display element may be driven as follows. Theplurality of scanning electrodes may be scanned at an interval of oneelectrode or plural electrodes, and remaining electrodes may be scannedin the same manner. When scanning is conducted at an interval of pluralelectrodes, the cycle is successively repeated, namely an interlacedriving may be performed.

[0083] The plural scanning electrodes may be selectively andsuccessively scanned in each frame, namely a progressive (non-interlace)driving may be conducted. In the case of interlace driving in which oneframe is separated into plural fields, the polarity of selection signalvoltage to be applied to the scanning electrode in each field may bereversed in every field.

[0084] Embodiments of the liquid crystal display apparatus will bedescribed with reference to the accompanying drawings.

[0085] (Liquid Crystal Display Element, see FIG. 1)

[0086] First, a liquid crystal display element which includes a liquidcrystal exhibiting a cholesteric phase (cholesteric characteristics)will be described.

[0087]FIG. 1 is a sectional view schematically showing a structure of areflective/laminate type full-color liquid crystal display element whichcan be driven by simple matrix driving method.

[0088] The liquid crystal display element 100 shown in FIG. 1 comprisesa light absorbing layer 121, a red display layer 111R lying on the layer121 and capable of performing display by switching a red selectivereflective state to a transparent state and vice versa; a green displaylayer 111G lying on the layer 111R and capable of performing display byswitching a green selective reflective state to a transparent state andvice versa; and a blue display layer 111B lying on the layer 111G andcapable of performing display by switching a blue selective reflectivestate to a transparent state and vice versa.

[0089] Each of the display layers 111R, 111G, 111B includes resin columnstructures 115, a liquid crystal 116 and spacers 117 between a pair oftransparent substrates 112 having transparent electrodes 113, 114.Insulating films 118 and orientation-controlling films 119 are formed onthe transparent electrodes 113, 114 when so required.

[0090] A seal material 120 is provided to seal the liquid crystal 116 ata periphery of the space between the substrates 112 (outside the displayregion).

[0091] The transparent electrodes 113, 114 are connected to a scanningdriving IC 131 and a signal driving IC 132 (see FIG. 2), respectively,and a predetermined pulse voltage is applied to the transparentelectrodes 113, 114, respectively. In response to the applied voltage,the display of the liquid crystal 116 is switched between a transparentstate which passes visible light therethrough and a selective reflectivestate which selectively reflects visible light of specific wavelengths.

[0092] The transparent electrodes 113 formed in the display layers 111R,111G, 111B, respectively are a plurality of strip electrodes extendingin parallel with each other with a minute space away from each other.The transparent electrodes 114 formed in the display layers 111R, 111G,111B, respectively are also a plurality of strip electrodes extending inparallel with each other with a minute space away from each other.

[0093] The transparent electrodes 113, 114 are opposed to each other ina direction orthogonal to each other when viewed on a plane. Voltagesare successively applied to the upper and lower strip electrodes. Namelya voltage is successively applied to the liquid crystal 116 in a matrixmanner to display an image. This method is called matrix driving. Eachpixel corresponds to a portion at which the electrode 113 and theelectrode 114 cross each other. Such matrix driving is conducted on eachdisplay layer, whereby a full color image can be displayed in the liquidcrystal display element 100.

[0094] Generally speaking, in a liquid crystal display element with aliquid crystal exhibiting a cholesteric phase between two substrates,the liquid crystal is switched between a planar state and a focal conicstate to display an image. When the liquid crystal is in the planarstate, a light of wavelength λ=P·n (wherein P is a helical pitch of thecholesteric liquid crystal and n is an average refractive index of theliquid crystal) is selectively reflected. When the liquid crystal is inthe focal conic state, light incident on the liquid crystal is scatteredin the case of selective reflective wavelength of the cholesteric liquidcrystal being in a range of infrared light. When the selectivereflective wavelength of the cholesteric liquid crystal is shorter,light is less scattered and visible light substantially passes throughthe liquid crystal.

[0095] Consequently, when a selective reflective wavelength is set in avisible light range and a light absorbing layer is formed on a sideopposite to the observation side of the element, a selective relectivecolor is displayed in the planar state while a black display can be donein the focal conic state.

[0096] When a selective reflective wavelength is set at an infraredlight range and a light absorbing layer is formed on a side opposed tothe observation side of the element, whereby light of wavelength in theinfrared light range is reflected while light of wavelength in visiblelight range passes therethrough in the planar state, so that a blackcolor can be displayed. A white color can be displayed due to lightscattering in the focal conic state.

[0097] In the liquid crystal display element 100 having the displaylayers 111R, 111G, 111B superposed on each other, when the blue displaylayer 111B and the green display layer 111G are brought to a transparentstate wherein liquid crystal molecules are in a focal conic arrangement,and the red display layer 111R is brought to a selective reflectivestate wherein liquid crystal molecules are in a planar arrangement, ared display can be performed. The blue display layer 111B is brought toa transparent state wherein liquid crystal molecules are in a focalconic arrangement, and the green display layer 111G and the red displaylayer 111R are brought to a selective reflective state wherein liquidcrystal molecules are in a planar arrangement, whereby a yellow displaycan be performed. Similarly the transparent state or selectivereflective state is suitably selected as the state of each displaylayer, whereby red, green, blue, white, cyan, magenta, yellow or blackcolor can be displayed.

[0098] Moreover, when an intermediate selective reflective state isselected as the state of display layers 111R, 111G, 111B, anintermediate color can be displayed and can be utilized for full colordisplay.

[0099] A liquid crystal exhibiting a cholesteric phase (cholestericcharacteristics) at room temperature can be preferably used as theliquid crystal 116. Especially it is suitable to use a chiral nematicliquid crystal prepared by adding a chiral material to a nematic liquidcrystal in an amount sufficient to show a cholesteric phase.

[0100] The chiral material is an additive which is capable of twistingthe molecules of nematic liquid crystal when added to the nematic liquidcrystal. The nematic liquid crystal is imparted a helical structure oftwisted molecules of liquid crystal by addition of the chiral materialto the nematic liquid crystal, whereby it is caused to show acholesteric phase.

[0101] The structure of liquid crystal display layer is not necessarilylimited to the above. A resin structure in the form of a wall or thelike may be used instead of the column structure 115, or such resinstructure may be omitted. Useful structures of the liquid crystal layerinclude conventional structures such as a layer structure wherein aliquid crystal is dispersed in a three-dimensional polymer network, alayer structure wherein a three-dimensional polymer network is formed ina liquid crystal (so-called polymer-dispersed type liquid crystalcomposite film) and the like.

[0102] (Driving Circuit, see FIGS. 2 and 3)

[0103]FIG. 2 is a block diagram showing an example of a driving circuitwhich is a main part of a driving device for applying driving voltagesto the liquid crystal display layer. FIG. 3 shows an example of adetailed structure of the driving circuit shown in FIG. 2. A logicalpower source and a logical level shifter shown in FIG. 3 are omitted inFIG. 2.

[0104] The liquid crystal display apparatus comprises the liquid crystaldisplay element 100 and the driving device shown in FIGS. 2 and 3.

[0105] According to the illustrated liquid crystal display apparatus,driving ICs 131, 132 are controlled by an LCD controller 136 based onimage data stored in an image memory 138 included in a controller CONTto be described later. Voltages are successively applied betweenscanning electrodes and signal electrodes in the liquid crystal displayelement 100, whereby an image is written in the liquid crystal displayelement 100.

[0106]FIGS. 2 and 3 and FIGS. 4 and 5 to be described later show thedriving ICs 131, 132 provided in any of the red, green and blue displaylayers. More specifically, the driving ICs 131, 132 are actuallyprovided in each of the red, green and blue display layers. The drivingICs 131, 132 are preferably provided in each of the red, green and bluedisplay layers (namely ICs are provided in three kinds of layers,respectively). It is possible to use any one of driving ICs 131, 132 incommon with these layers.

[0107] The driving device shown in FIGS. 2 and 3 include the scanningdriving IC (driver) 131, the signal driving IC (driver) 132, thecontroller CONT and a power source 140.

[0108] The controller CONT is provided with a central processing unit(CPU) 135 adapted to control the driving device in its entirety, the LCDcontroller 136 adapted to control the driving ICs, an image processingunit 137 for processing image data in various manners, and the imagememory 138 for storing image data. A power is supplied to the controllerCONT from a power source 140. The controller CONT is connected to thesignal driving IC 132 and, via a logical level shifter, to the scanningdriving IC 131. The logical level shifter is a circuit adapted to shifta ground(GND) potential to 0V for compensation if the ground(GND)potential is changed from 0V despite the ground (GND) to be kept at 0Vcorresponding to voltages to be supplied to the scanning driving IC. TheLCD controller 136 drives each driving IC according to the image datastored in the memory 138 based on directions from the CPU 135.

[0109] The pixel arrangement of the liquid crystal display element 100is represented by a matrix comprising the plurality of scanningelectrodes 113 (R1, R2 . . . Rm in FIG. 2) and the plurality of signalelectrodes 114 (C1, C2 . . . Cn in FIG. 2) (“m” and “n” being a naturalnumber) as shown in FIG. 2. The scanning electrodes R1, R2 . . . Rm areconnected to output terminals of the scanning driving IC 131, and thesignal electrodes C1, C2 . . . Cn are connected to output terminals ofthe signal driving IC 132.

[0110] The scanning driving IC 131 is connected to the scanningelectrodes R1, R2 . . . Rm as described above, to the controller CONTand to the power source 140. The driving IC 131 applies a drivingvoltage including a reset voltage (+V1 or −V1), a selection signalvoltage (+V2 or −V2) and a maintaining voltage (+V3 or −V3)) to thescanning electrodes R1, R2 . . . Rm.

[0111] The reset voltage is output, for example, as a positive resetpulse +V1 of +40V or a negative reset pulse −V1 of −40V. The selectionsignal voltage is output, for example, as a positive selection pulse +V2of +15V or a negative selection pulse −V2 of −15V, while the maintainingvoltage is output, for example, as a positive maintaining pulse +V3 of+25V or a negative maintaining pulse −V3 of −25V. These voltages areoutput from the scanning driving IC 131.

[0112] Voltage stabilizing condensers C connected to the ground(GND)corresponding to said voltages are connected to connection lines forsupplying the voltages +V1, +V2 and +V3, and −V1, −V2 and −V3 to thescanning electrodes 113. The logical power source connected to thescanning driving IC 131 is provided for supply of power to the scanningdriving IC 131.

[0113] The signal driving IC 132 is connected, as described above, tothe signal electrodes C1, C2 . . . Cn, to the controller CONT and to thepower source 140. A voltage (rewriting signal voltage (+V4, −V4)) outputfrom the power source 140 according to directions from the controllerCONT is applied to the signal electrodes C1, C2 . . . Cn, respectively.

[0114] The rewriting signal voltage is output as positive signal pulses+V4 of +3V and negative signal pulses −V4 of −3V from the signal drivingIC 132.

[0115] Voltage stabilizing condensers C connected to a ground(GND)corresponding to said voltages are connected to connection lines forsupplying the driving voltage (+V4, −V4) to the signal electrodes.

[0116] More specifically stated, the scanning driving IC 131 outputs theselection signal voltage to predetermined one among the scanningelectrodes R1, R2 . . . Rm to bring it to a selective state while itoutputs non-selection signals to other electrodes under directions fromthe controller CONT to bring them to a non-selective state. The scanningdriving IC 131 successively applies the selection signal voltage to thescanning electrodes R1, R2 . . . Rm, while switching the electrodes. Theapplication of the selection signal voltage to one scanning electrode isperformed in a scanning period set for the scanning electrode.

[0117] On the other hand, the signal driving IC 132 simultaneouslyoutputs the signals (rewriting signal voltages) corresponding to theimage data to the signal electrodes C1, C2 . . . Cn according todirections from the controller CONT to rewrite each pixel on thescanning electrode in the selective state. For example, if a scanningelectrode Ra is selected (“a” of the Ra is a natural number satisfying“a”≦m), pixels LRa-C1 . . . LRa-Cn corresponding to intersectionsbetween the scanning electrode Ra and the signal electrodes C1, C2 . . .Cn are rewritten at the same time. A voltage difference between theselection pulse voltage (selection signal voltage) applied to thescanning electrode and the signal pulse voltage (rewriting signalvoltage) applied to the signal electrode in each pixel is a voltage forrewriting the pixel so that the pixel is rewritten according to thevoltage.

[0118] The controller CONT is adapted to control the scanning driving IC131 such that the driving voltage to be applied to the scanningelectrodes R1, R2 . . . Rm in scanning operation in each frame formatrix driving of the liquid crystal display element 100 has a singlepolarity in each frame and the polarity of the driving voltage isreversed in every frame.

[0119] More specifically stated, when scanning is performed inodd-numbered frames, the scanning driving IC 131 successively appliesthe positive reset pulse voltage +V1, the positive selection pulsevoltage +V2 and the positive maintaining pulse voltage +V3 to eachscanning electrode R1, R2 . . . Rm while the signal driving IC 132applies the signal pulse ±V4 to each signal electrode C1, C2 . . . Cn.

[0120] When scanning is performed in even-numbered frames, the scanningdriving IC 131 successively applies the negative reset pulse voltage−V1, the negative selection pulse voltage −V2 and the negativemaintaining pulse voltage −V3 to each scanning electrode R1, R2 . . . Rmwhile the signal driving IC 132 applies the signal pulse ±V4 to eachsignal electrode C1, C2 . . . Cn (see FIGS. 6 to 8).

[0121] In the foregoing operation, the application period Tsp of theselection pulse voltage (selection signal voltage)(+V2 or −V2) is ½ thescanning period Tss and the signal pulse ±V4 is a voltage which ischanged in polarity within the scanning period Tss and effective valuesof positive and negative voltages thereof are substantially equal toeach other within the scanning period Tss.

[0122] Further the signal pulse is such that each of total of period(s)of the positive voltage and total of period(s) of the negative voltagewithin the scanning period Tss is as long as the application period Tspof the selection pulse.

[0123] As described above, the controller CONT controls the scanningdriving IC 131 such that the application period Tsp of the selectionpulse (+V2 or −V2) is ½ the scanning period Tss and controls the signaldriving IC 132 such that the signal pulse ±V4 is a voltage which ischanged in polarity within the scanning period Tss; the effective valuesof the positive and negative voltages of the signal pulse aresubstantially equal to each other within the scanning period Tss; andthe signal pulse is such that each of total of period(s) of the positivevoltage and total of period(s) of the negative voltage within thescanning period is as long as the application period of selection pulse(+V2, −V2). This matter will be described in more detail in respect ofdriving principle and example of basic driving.

[0124] The signal pulse voltage±V4 is a rectangular pulse voltage whichhas a duty ratio of 50% and the absolute values of positive and negativevoltages (+V4, −V4) are identical with each other.

[0125] In this driving device, the power source 140 can supply bothpositive and negative voltages at least all the time during drivingoperation. The driving voltage is applied to the scanning electrodes R1,R2 . . . Rm by the scanning driving IC connected to the power source140.

[0126] However, the supply of power is not limited to the above. Thedriving voltage may be applied to the scanning electrodes R1, R2 . . .Rm by the scanning driving IC connected to a power source which canswitch output voltages from positive to negative and vice versa.

[0127]FIGS. 4 and 5 show another example of structure of a drivingcircuit. In the structure of the circuit shown in FIGS. 4 and 5, a powersource switching circuit 141 is provided between the power source 140and the scanning driving IC in the circuit structure shown in FIG. 3.

[0128] In the structure of the circuit shown in FIGS. 4 and 5, the powersource 140 and the power source switching circuit 141 constitutes apower source 140′ which can switch positive and negative of outputvoltage.

[0129] The power source 140′ is connected to the controller CONT and has4 switching elements SW1 to SW4.

[0130] The elements SW1 to SW4 can be simultaneously switched underdirections from the controller CONT to a state of applying a positivedriving voltage (side 1 in the drawing) or to a state of applying anegative driving voltage (side 2 in the drawing). When the switchingelements are in the state of side 1, the power source 140′ can supplypositive voltages +V1, +V2, +V3 from the power source 140 to thescanning driving IC 131. On the other hand, when the switching elementsare in the state of side 2, the power source 140′ can supply negativevoltages −V1, −V2, −V3 from the power source 140 to the scanning drivingIC 131.

[0131] In the driving device having the circuit structure shown in FIGS.4 and 5, the controller CONT can control the power source 140′ and thescanning driving IC 131 so that the driving voltage to be applied to thescanning electrodes 113 by switching from positive voltages +V1, +V2,+V3 to negative voltages −V1, −V2, −V3 or vice versa is given a singlepolarity in each frame, and polarity inversion is effected in everyframe. According to the driving device, the driving of liquid crystaldisplay element can be realized by a simple circuit structure. FIG. 4shows the state of odd-numbered frames (plus frames) in which theswitching elements SW1 to SW4 are switched to the side 1. FIG. 5 showsthe state of even-numbered frames (minus frames) in which the elementsSW1 to SW4 are switched to the side 2.

[0132] An image can be rewritten usually by successively selecting allscanning lines. When an image is partially rewritten, specific scanninglines alone are successively selected in a way to include a part to berewritten. Thereby only the required part can be rewritten in a shorttime. In the circuit structure shown in FIGS. 4 and 5, the voltages tobe supplied to the scanning driving IC is ½ the voltages in thestructure in FIG. 3. Consequently the scanning driving IC which isinexpensive and which is relatively low in voltage resistance ascompared with the structure of FIG. 3 can be used.

[0133] (Driving Principle and an Example of Basic Driving, see FIGS. 6to 11)

[0134] The basic principle of the method of driving the liquid crystaldisplay element 100 is first described. Hereinafter, this matter isexplained with reference to specific example using pulse waveforms.However, the driving method is not limited to these waveforms.

[0135]FIG. 6(A) shows an example of basic driving waveform inodd-numbered frame (plus frame) which is output from the scanningdriving IC 131 to each scanning electrode, and FIG. 6(B) shows anexample of basic driving waveform in even-numbered frame (minus frame)which is output from the scanning driving IC 131 to each scanningelectrode.

[0136]FIGS. 7 and 8 show waveforms of voltages which are output from thescanning driving IC 131 to each scanning electrode 113 (row electrode),a waveform of voltage which is output from the signal driving IC 132 toone signal electrode (column electrode), and waveforms of voltages asapplied to the liquid crystals (indicated as LCD 1 to LCD 28 in thedrawing) corresponding to pixels by these voltages. FIG. 7 showswaveforms of voltages in odd-numbered frame, and FIG. 8 shows waveformsof voltages in even-numbered frame.

[0137]FIGS. 7 and 8 indicate an example of basic driving in which aselection pulse voltage (selection signal voltage) is successivelyoutput to the plurality of scanning electrodes 113 (illustrated as 28row electrodes 1, 2-28 in the drawings) and a signal pulse (rewritingsignal voltage) is output from one signal electrode (depicted as acolumn b in the drawings, the “b” being a natural number satisfying b≦n)which is one of the plurality of signal electrodes 114 (a plurality ofcolumn electrodes).

[0138] The waveform of signal pulse output from the column b shown inthe drawings is a waveform capable of successively outputting a pulsewhich selects the selective reflective state of the liquid crystal inany of scanning periods Tss. It is possible to output any of a waveformof signal pulse selecting a transparent state, a waveform of signalpulse selecting a selective reflective state and a waveform of signalpulse selecting a mixed state (mixture of these states) from the columnb. This matter will be described in more detail later.

[0139] Indicated at LCD 1, 2 to 28 in the drawings are liquid crystalscorresponding to the pixels intersectionally formed between the scanningelectrodes (rows 1, 2-28) and the signal electrode (column b), and arewaveforms of voltages applied to the liquid crystals corresponding tothe pixels. A cross-talk pulse due to the signal pulse applied to thesignal electrode is applied to the liquid crystals. FIGS. 7 and 8indicate, in thick lines, ranges to which the cross-talk pulse isapplied. This matter will be explained in detail later.

[0140] In this driving, as described above, the driving voltage to beapplied to the scanning electrodes (rows 1, 2 to 28) in scanning isgiven a single polarity in each frame and the polarity is reversed inevery frame. For example, the driving voltage is given a single polarityin scanning in one frame, namely until the scanning operation in oneframe is completed, using the first scanning electrode (row 1) to thelast scanning electrode (row 28). Then the polarity of the drivingvoltage is reversed for scanning in next one frame.

[0141] A driving period is roughly divided into a reset period Trs, aselection period Ts, a maintaining period Trt and a display period Ti.The selection period Ts is subdivided into a selection pulse applicationperiod (application period of selection signal voltage) Tsp, apre-selection period Tsz and a post-selection period Tsz′. The resetperiod Trs and the maintaining period Trt are, for instance, 48 ms. Theselection period Ts is, for example, 0.6 ms. The pre-selection periodTsz and the post-selection period Tsz′ are both, for example, ⅓ (0.2 ms)the selection period Ts (0.6 ms). The scanning period Tss is ⅓ (0.2 ms)the selection period Ts (0.6 ms). The selection pulse application periodTsp is ½ (0.1 ms) the scanning period Tss as described above.

[0142] As illustrated in FIGS. 6 to 8, in basic driving waveforms, areset pulse (positive pulse +V1 in odd-numbered frames and negativepulse −V1 in even-numbered frames) is applied in the reset period Trs.In the selection period Ts, a selection pulse (positive pulse +V2 inodd-numbered frames and negative pulse −V2 in even-numbered frames) isapplied in the selection pulse application period Tsp. In the scanningperiod Tss including the period Tsp, a signal pulse ±V4 is applied fromthe signal driving IC 132. The signal pulse ±V4 is determined based onthe image data. As described above, the signal pulse ±V4 is arectangular pulse which has a duty ratio of 50% and in which theabsolute values of positive and negative voltages (+V4, −V4) areidentical with each other. In the basic driving waveform, the voltage iszero in the pre-selection period Tsz and the post-selection period Tsz′.Further, a maintaining pulse (positive pulse +V3 in odd-numbered frames,and negative pulse −V3 in even-numbered frames) is applied in themaintaining period Trt.

[0143] The liquid crystal operates as follows. First, when the resetpulse of +V1 (odd-numbered frames) or −V1 (even-numbered frames) isapplied in the reset period Trs, the liquid crystal is reset to ahomeotropic state. The reset period Trs proceeds to the selection pulseapplication period Tsp via the pre-selection period Tsz (during whichthe liquid crystal becomes slightly retwisted). The waveform of thepulse to be applied to the liquid crystal in the period Tsp is variedwith a pixel finally selecting a planar state or with a pixel finallyselecting a focal conic state.

[0144] FIGS. 6 to 8 show cases of selecting a planar state. When a focalconic state is to be selected, the phase of the signal pulse is shiftedto an extent corresponding to a half-period compared with the case ofselecting a planar state.

[0145] The case of selecting a planar state will be described. In thiscase, in the selection pulse application period Tsp, a voltage of[(+V2)−(−V4)] in odd-numbered frames or a voltage of [(−V2)−(+V4)] ineven-numbered frames is applied to the liquid crystal to bring theliquid crystal to a homeotropic state again. Thereafter the liquidcrystal becomes slightly retwisted in the post-selection period Tsz′.Then when the maintaining pulse of +V3 (odd-numbered frames) or −V3(even-numbered frames) is applied in the maintaining period Trt, theliquid crystal having become slightly retwisted in the post-selectionperiod Tsz′ becomes further loose by application of the maintainingpulse and is brought to a homeotropic state.

[0146] The liquid crystal in the homeotroic state is brought to a planarstate by change-over to voltage zero and is fixed in the planar state.

[0147] On the other hand, when a focal conic state is finally selected,a voltage of [(+V2)−(+V4)] in odd-numbered frames or a voltage of[(−V2)−(−V4)] in even-numbered frames is applied in the selection pulseapplication period Tsp. In post-selection period Tsz′, the liquidcrystal becomes retwisted and a state having a helical pitch spreadingapproximately twice.

[0148] Subsequently, the maintaining pulse of +V3 (odd-numbered frames)or −V3 (even-numbered frames) is applied in the maintaining period Trt.The liquid crystal having become slightly retwisted in thepost-selection period Tsz′ is brought to a focal conic state byapplication of the maintaining pulse. The liquid crystal in the focalconic state is fixed in the focal conic state even by change-over tovoltage zero.

[0149] According to the above-described method and device for drivingthe liquid crystal display element and liquid crystal display apparatus,when the scanning operation is performed in each frame for matrixdriving of the liquid crystal display element 100, the driving voltageto be applied to the scanning electrodes 113 is given a single polarityin each frame and the polarity is reversed in every frame, whereby thestate of single polarity of the voltage to be applied to the liquidcrystal 116 in each frame can continuously last for a prolonged periodof time. Consequently compared with use of, for example, an alternatingvoltage whose polarity of voltage waveform is periodically changed as avoltage to be applied to the liquid crystal 116, it is possible toreduce a waveform repeating frequency of voltage to be applied to theliquid crystal 116, and the value of driving voltage to be applied tothe scanning electrode 113 can be decreased by ½, therebycorrespondingly lowering the consumption of power for driving the liquidcrystal display element 100. Namely the liquid crystal display element100 can be driven by reduced power consumption.

[0150] As set forth above, the final display state of the liquid crystalcan be selected by the pulse to be applied to the liquid crystal in theselection pulse application period Tsp. An intermediate tone display canbe achieved by adjusting a voltage value of pulse, pulse width and/orphase, more specifically by adjusting the waveform of the signal pulseto be applied to the signal electrode in accordance with image data.

[0151] The following description will be made about an intermediate tonedisplay performed by changing a phase of the signal pulse to be appliedto the signal electrode.

[0152] FIGS. 9 to 11 show a waveform of the selection pulse voltagewhich is output to a single row a selected from the rows 1 to 28, awaveform of the signal pulse voltage which is output to the column b anda waveform of a voltage applied to the liquid crystal LCDx by theselection pulse voltage and the signal pulse voltage in odd-numberedframes (plus frames).

[0153]FIG. 9 indicates waveforms for finally setting the liquid crystalin a maximum selective reflective state. FIG. 10 illustrates waveformsfor finally setting the liquid crystal in an intermediate tone displaystate. FIG. 11 shows waveforms for finally setting the liquid crystal ina transparent state.

[0154]FIG. 12(A), FIG. 12(B), and FIG. 12(C) show the waveform of theselection pulse voltage to be output to the row a, the waveform of thesignal pulse voltage to be output to the column b, and the waveform ofthe voltage to be applied to the liquid crystal LCDx, chiefly those inthe selection period, these views being enlarged from those shown inFIG. 10, FIG. 9 and FIG. 11, respectively.

[0155] As shown in FIG. 12(A) to FIG. 12(C), the signal pulse of ±V4 tobe applied to the column b is such that a total of minus period(s) (ortotal of plus period(s)) of the signal pulse voltage during the signalpulse application period Tw (scanning period Tss) is as long as theselection pulse application period Tsp. Accordingly, it is possible thata rise or fall timing of the signal pulse to be applied to the column bin synchronization with application of the selection pulse to the row ais shifted within a period from zero to the Tsp (½ the scanning periodTss) (see t1 (or t2) in FIG. 12(A)), thereby changing a width of each ofhighest portion |V2+V4| and lowest portion |V2−V4| of the pulse voltageto be applied to the liquid crystal LCDx within the selection pulseapplication period and controlling the final state of the liquidcrystal.

[0156] As shown in FIG. 12(A), in the waveform of the voltage forselecting the intermediate tone display state of the liquid crystal, atotal of application periods of highest portion |V2+V4| and lowestportion |V2−V4| of the pulse voltage to be applied to the liquid crystalLCDx within the selection pulse application period is equal to theselection pulse application period Tsp. As shown in FIG. 12(B), in thewaveform of the voltage for selecting the selective reflective state ofthe liquid crystal, a width of the highest portion |V2+V4| of the pulsevoltage to be applied to the liquid crystal LCDx within the selectionpulse application period is equal to the width of the selection pulseapplication period Tsp. As shown in FIG. 12(C), in the waveform ofvoltage for selecting the transparent state of the liquid crystal, awidth of the lowest portion |V2−V4| of the pulse voltage to be appliedto the liquid crystal LCDx within the selection pulse application periodis equal to the width of the selection pulse application period Tsp.

[0157] The operation in the minus frame in which a minus voltage isapplied to the rows is the same in the plus frame except that thepolarity of the voltage to be applied to the rows and the polarity ofthe voltage to be applied to the columns are reversed.

[0158] As indicated with thick lines in FIGS. 7 and 8, any of thewaveforms of the voltages to be applied to the liquid crystals LCD1,LCD2 suffers a cross-talk due to the signal pulse to be applied to thesignal electrode.

[0159] However, the waveform of the signal pulse to be applied to thecolumn b is a rectangular pulse waveform which has a duty ratio of 50%and in which the absolute values of positive and negative voltages areidentical with each other. In any waveform for selecting a display stateof the liquid crystal, the voltage applied to the liquid crystal in thereset period and the voltage applied to the liquid crystal in themaintaining period are constant, respectively as shown in FIG. 12(A) toFIG. 12(C), since an effective voltage is {square root}{square root over( )}{[(V1+V4)²+(V1−V4)²]/2} in the reset period and an effective voltageis {square root}{square root over ( )}{[(V3+V4)²+(V3−V4)²]/2} in themaintaining period.

[0160] As described above, if the selection pulse voltage to be appliedto the row a is a voltage whose application period Tsp is ½ the scanningperiod Tss and the signal pulse voltage to be applied to the column bhas a rectangular pulse waveform which has a duty ratio of 50% and inwhich the absolute values of positive and negative voltages areidentical with each other, voltages applied to the liquid crystals LCD1, LCD 2 to LCD 28 corresponding to pixels due to the cross-talk can bemade substantially constant, whereby a shadowing occurring in imagedisplay due to the cross-talk can be suppressed.

[0161] The foregoing embodiments use a signal pulse which has a dutyratio of 50% and in which a positive or negative continuous time is aslong as Tsp. However, if a duty ratio is 50%, an effect of suppressingthe shadowing can be achieved without the feature that a positive ornegative continuous time is as long as as Tsp. In that case, the signalpulse may be output as separated into two while maintaining a duty ratioat 50%.

[0162] However, this case involves twice the inversion frequency ofsignal pulse and twice the power consumption of the display element(display panel) arising from the signal pulse. FIG. 13(A) to FIG. 13(C)show an example of signal pulse wherein continuous time T1 or T2 ofpositive or negative voltage is not as long as the selection signalapplication period Tsp. In this example, intermediate tone display statecan be selected by applying signal pulse shown in FIG. 13(A) in whichtotal sum of the continuous times T1 and T2 is kept to the period Tsp.By varying the ratio of the continuous times T1 and T2 with keeping thesum of the times T1 and T2 to the period Tsp, various tone displaystates including the maximum selective reflective display state (theplanar state: T1=0, T2=Tsp: FIG. 13(B)) and the transparent state (thefocal conic state: T1=Tsp, T2=0: FIG. 13(C)) can be selected.

[0163] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and the scope of the present invention beinglimited only by the terms of the appended claims.

What is claimed is:
 1. A liquid crystal display apparatus comprising: aliquid crystal display element that includes a layer of a liquid crystalexhibiting a cholesteric phase, and a plurality of scanning and signalelectrodes extending across each other with the liquid crystal layertherebetween for performing display utilizing a selective reflectioncapability of the liquid crystal; and a driving device for driving theliquid crystal display element by simple matrix driving, wherein thedriving device is configured such that (1) a driving voltage of singlepolarity including a selection signal voltage is applied to the scanningelectrodes in each frame, and the polarity of the driving voltage isreversed in every frame; (2) the scanning electrodes are successivelybrought to a selected state by applying the selection signal voltage toeach scanning electrode in a scanning period set for the scanningelectrode, while a rewriting signal voltage corresponding to eachscanning electrode in the selected state is applied to each signalelectrode in synchronization with application of the selection signalvoltage to the scanning electrode; and (3) an application period of theselection signal voltage to the scanning electrode is half of thescanning period.
 2. The liquid crystal display apparatus according toclaim 1, wherein the rewriting signal voltage to be applied to thesignal electrode is changed in polarity within the scanning period andeffective values of positive and negative voltages of the rewritingsignal voltage are substantially equal to each other within the scanningperiod.
 3. The liquid crystal display apparatus according to claim 1,wherein the rewriting signal voltage to be applied to the signalelectrode is such that sum total of total of period(s) of positivevoltage and total of period(s) of negative voltage is as long as thescanning period.
 4. The liquid crystal display apparatus according toclaim 1, wherein the driving device is capable of varying a density ofan image to be finally displayed by adjusting a phase of the rewritingsignal voltage to be applied to the signal electrode in synchronizationwith application of the selection signal voltage to the scanningelectrode.
 5. The liquid crystal display apparatus according to claim 1,wherein the liquid crystal display element can maintain a displaywithout application of voltage.
 6. The liquid crystal display apparatusaccording to claim 1, wherein the driving device applies a reset voltageto each scanning electrode for bringing the liquid crystal to ahomeotropic state before a selection period while the selection signalvoltage is applied.
 7. The liquid crystal display apparatus according toclaim 6, wherein the driving device applies a maintaining voltage toeach scanning electrode for eastablishing a state of the liquid crystalto be selected by the selection signal voltage after application of theselection signal voltage to the scanning electrode.
 8. The liquidcrystal display apparatus according to claim 1, wherein the drivingdevice performs an interlace driving in which one frame is separatedinto a plurality of fields.
 9. The liquid crystal display apparatusaccording to claim 1, wherein the driving device performs a progressivedriving in which the plurality of scanning electrodes are succesivelyscanned in each frame.
 10. The liquid crystal display apparatusaccording to claim 1, wherein the driving device includes a powercircuit which can switch positive and negative of output voltage and ascanning driving IC connected between the power circuit and the pluralscanning electrodes, and wherein the polarity of the driving voltage tobe applied to the scanning electrode can be reversed by change-over ofthe positive and negative of output voltage of the power circuit inevery frame.
 11. The liquid crystal display apparatus according to claim10, wherein the power circuit includes a power source having a pluralityof output terminals and a circuit for switching the terminals to beconnected to the scanning driving IC.
 12. The liquid crystal displayapparatus according to claim 1, wherein the liquid crystal displayelement is a laminate type element having a plurality of laminatedliquid crystal layers, wherein the driving device has a scanning drivingIC connected to the scanning electrodes, and a signal driving ICconnected to the signal electrodes, and wherein any one of the scanningdriving IC and the signal driving IC is used in common for driving eachof the liquid crystal layers.
 13. A liquid crystal display apparatuscomprising: a liquid crystal display element that includes a layer of aliquid crystal exhibiting a cholesteric phase, and a plurality ofscanning and signal electrodes extending across each other with theliquid crystal layer therebetween; and a driving device for driving theliquid crystal display element by simple matrix driving, wherein thedriving device is configured such that (1) a driving voltage of singlepolarity including a selection signal voltage, a reset voltage and amaintaining voltage is applied to the scanning electrodes in each frame,and the polarity of the driving voltage is reversed in every frame; (2)the scanning electrodes are successively brought to a selected state byapplying the selection signal voltage to each scanning electrode in ascanning period set for the scanning electrode, while a rewriting signalvoltage corresponding to each scanning electrode in the selected stateis applied to each signal electrode in synchronization with applicationof the selection signal voltage to the scanning electrode; (3) the resetvoltage is applied to the scanning electrode to bring the liquid crystalto a homeotropic state before applying the selection signal voltage, andthe maintaining voltage is applied to the scanning electrode toestablish a state of the liquid crystal to be selected by the selectionsignal voltage after applying the selection signal voltage; and (4) therewriting signal voltage to be applied to the signal electrode ischanged in polarity within the scanning period, and effective values ofpositive voltage and negative voltage of the rewriting signal voltageare substantially equal to each other within the scanning period. 14.The liquid crystal display apparatus according to claim 13, wherein theliquid crystal display element can maintain a display withoutapplication of voltage.
 15. The liquid crystal display apparatusaccording to claim 13, wherein the driving device is configured suchthat a period is set for bringing the voltage to be applied to thescanning electrode to 0V between application of the reset voltage andapplication of the selection signal voltage and/or between applicationof the maintaining voltage and application of the selection signalvoltage.
 16. The liquid crystal display apparatus according to claim 13,wherein the driving device is capable of varying a density of an imageto be finally displayed by adjusting a phase of the rewriting signalvoltage to be applied to the signal electrode in synchronization withapplication of the selection signal voltage to the scanning electrode.17. The liquid crystal display apparatus according to claim 13, whereinthe reset voltage and the maintaining voltage to be applied to thescanning electrode are different in voltage value from the selectionsignal voltage, respectively.
 18. A liquid crystal display apparatuscomprises a liquid crystal display element, and a driving device fordriving the liquid crystal display element by simple matrix driving,wherein the liquid crystal display element includes a liquid crystallayer, and has a plurality of scanning electrodes and a plurality ofsignal electrodes extending across each other with the liquid crystallayer therebetween, wherein the driving device is configured such that adriving voltage of single polarity including a selection signal voltageis applied to the scanning electrodes in each frame, and the polarity ofthe driving voltage is reversed in every frame; the scanning electrodesare successively brought to a selected state by applying the selectionsignal voltage to each scanning electrode in a scanning period set forthe scanning electrode, while a rewriting signal voltage correspondingto each scanning electrode in the selected state is applied to eachsignal electrode in synchronization with application of the selectionsignal voltage to the scanning electrode; and the rewriting signalvoltage to be applied to the signal electrode is changed in polaritywithin the scanning period, and effective values of positive voltage andnegative voltage of the rewriting signal voltage are substantially equalto each other within the scanning period.
 19. The liquid crystal displayapparatus according to claim 18, wherein the driving device applies therewriting signal voltage corresponding to each scanning electrode in theselected state to each signal electrode in synchronization withapplication of the selection signal voltage to the scanning electrode,and an application period of the selection signal voltage to thescanning electrode is ½ the scanning period.
 20. The liquid crystaldisplay apparatus according to claim 18, wherein the driving deviceincludes a power circuit which can switch positive and negative ofoutput voltage and a scanning driving IC connected between the powercircuit and the plural scanning electrodes, and the power circuitincludes a power source having a plurality of output terminals and acircuit for switching the terminals to be connected to the scanningdriving IC, and wherein the circuit for switching the terminals reversesthe polarity of the driving voltage to be applied to the scanningelectrode by changing the terminals to be connected to the scanningdriving IC in every frame.